Abstract: The test accuracy of soil heat flux at 5 mm is verified through thermal diffusion equation-correction (TDEC) method, which can represent the surface heat flux. Meanwhile, considering different factors (solar radiation, soil moisture content and cloud cover, etc.), the diurnal variations of heat flux on three kinds of typical days (clear, cloudy and rainy days) are summarized in Shanghai. The influence of different factors on soil heat flux at 5mm and the principles are comprehensively analyzed through variable control approaches. The results show that soil heat flux at 5 mm obviously fluctuates on clear days, from -23.62W/m2 to 111.81W/m2. The fluctuation range decreases with the decrease of soil depth on cloudy and rainy days. The correlation (R2>0.74) between solar radiation and soil heat flux is strong on days without rain. The fluctuation range of soil heat flux is reduced by the increase of soil moisture content. Soil heat flux of different typical days is mostly affected by different factors.

Key words: surface heat flux; solar radiation; soil moisture content; thermal diffusion equation-correction (TDEC) method; typical days